We have used 3T3-LI fibroblasts for studying hormonal regulation of lipoprotein lipase (LPL) in adipocytes. Earlier studies showed that insulin is most important in expression of the fat cell phenotype, development of LPL and deposition of triacylglycerol (TG). Insulin is also the primary regulatory factor of LPL in mature 3T3-LI fat cells. Immunochemical methods are now being developed to assay the enzyme protein independent of its catalytic activity. Preliminary results indicate that under certain conditions when there are rapid, large changes in enzyme activity in the cells, there is little or not change in the amount of immunoreactive protein. This suggests there are mechanisms for rapid interconversion of high and low activity forms of the enzyme in the cells.